1. Field of the Invention
The present invention relates to a piezoelectric displacing device which utilizes the longitudinal effect of a piezoelectric material.
2. Description of the Prior Art
In a longitudinal effect-type piezoelectric displacing device, when a signal voltage is applied to a piezoelectric element, the element expands/contracts in the direction along the axis of an electric field acting thereupon. Thus, if one end of the element is fixed in position, the other end thereof is displaced. The element of such a device generally consists of PZT or a ternary polycrystalline material. In order to prepare a piezoelectric displacing element, the piezoelectric material is formed into a column 1, and a DC high voltage is applied to the column along an axis parallel to the central axis of the column 1 for polarization along this axis, as shown in FIG. 1. The polarized element has a "polarization direction" along the axis parallel to the central axis, and exhibits piezoelectricity which depends on this polarization direction. When electrodes 2 are formed on two end faces of the column 1 and a signal voltage V.sub.1 is applied between the electrodes 2 so as to apply an electric field in the polarization direction, the element expands/contracts in the direction of the axis parallel to the central axis of the column 1 in correspondence with the polarity of the voltage applied. When a polarization axis P, an electric field axis E and a displacement axis .delta. coincide with each other, this is referred to as the "longitudinal effect" of a piezoelectric material.
For a given element of such a device, the amount of displacement of a longitudinal effect-type piezoelectric displacing device is proportional to the product of an intensity of an electric field (a ratio of the signal voltage applied between the electrodes in the example of the element described above to the thickness of the piezoelectric material between the electrodes) and the size of the element along the electric field axis, that is, the length of the column. For this reason, in order to increase the amount of displacement, two methods, firstly of increasing the intensity of the electric field and secondly of increasing the length of the element along the electric field axis, are conventionally adopted. When the first method is adopted, the amount of displacement, that is, change, reaches saturation, and the degree of polarization decreases depending upon the polarity of the electric field applied. Accordingly, the degree to which the intensity of the electric field may be increased is limited; it is usually 10 to 50% of the coersive field depending on the piezoelectric material.
When the second method as described above is adopted, a device mounting such an element with an increased length becomes large in size, thus preventing actual adoption of this method. Accordingly, the amount of displacement of a longitudinal effect-type piezoelectric displacing device is limited due to limits on the intensity of the electric field applied and the size of the device along the displacement axis. For example, when a ceramic piezoelectric material is used as a displacing element, the upper limit of the piezoelectric constant associated with displacement is about 500.times.10.sup.-12 m/V and the maximum intensity of the electric field within a normal operation range is about 1 kV/mm. In this case, if the column has a length of 10 cm, an amount of displacement of only about 50 .mu.m is obtained. Such a longitudinal effect-type piezoelectric displacing device can be applied to various types of measuring equipment, a precision displacing device such as a processor, or a precision drive apparatus such as an X-Y table. However, it is desired to increase the amount of displacement in such a device beyond the conventionally possible level without requiring an increase in the size of the element along the displacement axis, that is, the length of the element along the electric field axis.
Meanwhile, a piezoelectric displacing device is known to have an important defect. More specifically, displacement of an element has a hysteresis characteristic with respect to an applied electric field, so that the amount of displacement and the intensity of the electric field are not linear. For this reason, the amount of displacement of the displacing device cannot be correctly controlled. It has therefore been desired to reduce this hysteresis phenomenon, i.e., the residual displacement caused after the electric field is removed.
Such residual displacement may be reduced by forming a displacing element having a structure which provides an increased amount of displacement for a given applied electric field. This method is established based on the following. In a general operation range, the displacement of an element of a piezoelectric displacing device is substantially linearly proportional to the intensity of an electric field, and the residual displacement is substantially proportional to the power of .beta. (where .beta.&gt;1) of the electric field. FIG. 2 demonstrates this wherein the amount of displacement .delta. is plotted along the axis of ordinate and the intensity of the electric field E is plotted along the axis of abscissa. Curve a shows the relationship between the amount of displacement of the element and the intensity of the electric field. Curve b shows the relationship between the residual displacement and the intensity of the electric field. When the length of a displacing element utilizing the longitudinal effect is effectively increased to N times the original value and the expansion/contraction amount with respect to the intensity of the electric field is also increased to N times the original value, the proportionality constant of the amount of displacement and amount of residual displacement with respect to the intensity of the electric field is also increased to N times the original value. Accordingly, an intensity of the electric field for obtaining a desired amount of displacement is reduced to 1/N times the intensity of electric field E.sub.1 before displacement sensitivity (ratio of the amount of displacement to the intensity of the electric field) is increased to N times the original value. Accordingly, if the residual displacement after obtaining a given displacement is proportional to E.sub.1.sup..beta. before the displacement sensitivity is increased, it is proportional to N(E.sub.1 /N).sup..beta. after the displacement sensitivity is increased to N times the original value. This means that when the displacement sensitivity is increased to N times the original value, the residual displacement can be reduced to 1 /N.sup..beta.-1. It is concluded from this that if a small displacing device having a new structure is provided which has a constant element length along the displacement axis and which has an improved displacement sensitivity, a piezoelectric displacing device can be provided which is small in size, and is less subject to a hysteresis characteristic; that is, which has an excellent linear response characteristic of a displacement with respect to the intensity of an electric field.